1. Field of the Invention
The present invention relates to a method of anaerobicly sealing an open end of a capillary tube, which is filled with a liquid sample, such as a blood sample. The invention also relates to a closure cap for sealing an open end of such capillary tube.
2. Description of the Prior Art
Anaerobic sampling of blood by using a capillary tube is well known in the art and is i.a. described by Ole Siggaard-Andersen on page 150 of a publication entitled "The Acid-Base Status of the Blood", fourth edition, issued by Munksgaard, Copenhagen 1974.
When such a capillary tube sample is used for determining the socalled blood gas parameters, namely pH, the partial pressure of oxygen (pO.sub.2) and the partial pressure of carbon dioxide (pCO.sub.2), it is extremely important that the blood sample is treated anaerobicly in the period of time from the sample is taken till it is analyzed. Therefore, it is necessary to seal the ends of the capillary tube by means of suitable means immediately after sampling.
The capillary tube may, for example, be closed as described in connection with FIG. 26 on page 150 of the above publication. According to the method disclosed in the above publication the sampling is carried out by positioning a heparinized capillary tube in a horizontal position adjacent to a puncture, so that blood may flow from that puncture into the tube by capillary effect without coming into contact with the atmosphere.
When the tube has been filled completely, one end thereof is sealed by means of a plastic sealing material kept in a cup or a box. This sealing is carried out by pressing the respective open end of the capillary tube approximately 4 mm into the sealing material. The sealing material will then be pressed into the open end of the tube, and a corresponding volume of blood will be expelled from the opposite end of the capillary tube.
A short length of a steel wire is now positioned within the tube, and the other open end of the tube is sealed by pressing the said end approximately 2 mm into the sealing material. This causes the sealing material in the first open end of the capillary tube to be displaced a corresponding length outwardly provided that the tube is completely filled with blood without gas bubbles. Thus, it is important that the second end of the capillary tube is sealed by pressing this end into the sealing material to a depth which is smaller than that used in sealing the first end of the capillary tube.
The sealing material which is frequently used for that purpose is a putty-like material marketed by Radiometer A/S under the designation Sealing Wax D 553 943-800. This material is packed in a small box and intended for several uses. The sealing material has such a surface area that is may be used for sealing about 50 capillary tubes. When the known method described above is used it it not possible to avoid that residues of blood are left in the sealing material. However, in connection with blood analysis it should always be attempted to avoid such residues, or they must be removed as soon as possible because the existence of such blood residues mean a potential risk of infection. Therefore, it is desirable to provide an alternative method of sealing capillary tubes.
Such an alternative method involving the use of closure caps has already been proposed. However, as explained in the following this alternative method is also disadvantageous.
Commercially available closure caps, which are described more in detail below in connection with FIG. 3 of the drawings, are made from a flexible material, and each cap comprises a skirt portion having an inner diameter corresponding to the outer diameter of the capillary tube, apart from a possible enlarged part at the open end of the skirt portion for facilitating mounting of the cap on the capillary tube end. By using these known closure caps, expulsion of blood from the capillary tube during the sealing procedure may be substantially avoided. This eliminates the risk of contamination and also the risk that a seal established at the first end of a capillary tube is broken by establishing a seal at the opposite tube end.
However, it is rather difficult to seal the open end of a capillary tube anaerobicly by means of a closure cap of the known type, because the closure cap must be held by one hand and compressed between a pair of fingers so as to expell air from the inner space of the cap skirt portion before mounting. The capillary tube may then be gripped by the other hand, and one end of the tube may be pushed into the skirt portion of the closure cap while the compressive force exerted on the skirt portion by the fingers is slowly released so that the end of the capillary tube may be brought into engagement with the inner surface of the cap end wall substantially without inclusion of air. However, correct mounting of the known closure caps requires a certain skill and experience, because it is difficult to ascertain whether the tube end has been brought into proper engagement with the cap end wall, or whether a small air volume has been entrapped in a space defined within the skirt portion between the tube end and the cap end wall in a compressed condition. In the latter case no anaerob sealing of the capillary tube is obtained.
German Offenlegungsschrift No. 2,848,535 discloses a device for sampling of blood by means of a capillary tube having one end thereof closed by a stopper having venting passages therein communicating the inner space of the capillary tube with the ambient atmosphere. Thus, the inner space of the tube is vented so that air may escape from the capillary tube when it is filled with blood. Thus, the stopper does not seal the capillary tube anaerobicly, but causes that samples having a rather accurate predetermined volume may be taken out by means of this known sampling device.
Further prior art is disclosed in German Offenlegungsschrift No. 2,906,209, published Swedish patent specification No. 358,552, and German Auslegeschrift No. 2,455,631.